1. Technical Field
The present invention relates to devices for use with passive intermodulation (PIM) measuring instruments for improving measurement of distance to a fault-creating PIM.
2. Related Art
Passive intermodulation (PIM) causes an unwanted signal or signals to be generated by the non-linear mixing of two or more frequencies in a passive device such as a connector or cable. PIM has surfaced as a problem for cellular telephone technologies such as Global System for Mobile Communications (GSM), Advanced Wireless Service (AWS) and Personal Communication Service (PCS) systems. Cable assemblies connecting a base station to an antenna on a tower using these cellular systems typically have multiple connectors that can act as sources of PIMs that interfere with system operation.
PIM signals are created when two signals from different systems or the same system combine and are then reflected at a PIM point such as a faulty cable connector. If the generated PIM harmonic frequency components fall within the receive band of a base station, it can effectively block a channel and make the base station receiver think that a carrier is present when one is not. Generally the components of concern are third, fifth, and seventh order, where the third order is of greatest signal strength, and therefore, of primary concern. PIMs can, thus, occur when two base stations operating at different frequencies, such as an AWS device and a PCS device, are in close proximity.
PIMs can be reduced or eliminated by replacing faulty cables or connectors. Test systems are utilized to detect PIMs enabling a technician to locate the faulty cable or connector. The test system to measure PIMs creates signals at two different frequencies, amplifies them, and provides them through cables connecting a base station to antennas on a tower for the base stations. A return signal carrying the PIMs is filtered to select a desired test frequency harmonic where PIMs can be detected and the PIM and distance to PIM measurement is provided to an operator.
FIG. 1A is a block diagram of an exemplary prior art test system for measuring PIM. The test system includes a measuring instrument 1 that utilizes two signal sources, with a first signal source 2 producing a signal at frequency F1 and the second signal source 4 producing a signal at frequency F2. When these multiple signals are allowed to share the same signal path in a nonlinear transmission medium, the unwanted signals can occur. The third order response is particularly troublesome as it produces signals at 2F1-F2 as well as 2F2-F1. Test signals F1, F2 generated by the signal sources 2, 4 are provided to a combiner 6 to create a combined signal with frequencies F1 and F2 at the combiner output. A diplexer 8 sends the combined signal F1 and F2 to a test port 10 connected with a load and a PIM source 30. A reverse or reflected signal from the test port 10 is then produced at frequency 2F1-F2, and forwarded through the diplexer 8 to be down converted to an intermediate frequency. The reverse or reflected signal is output to a mixer 12, 22 where it is combined with a signal LO1, LO2 generated by a local oscillator (LO) 14, 24, and the target frequency or frequencies is selected by filtering the mixer output using a low-pass filter 16, 26. As shown, the reverse or reflected signal is down converted to a target intermediate frequency in two stages. The magnitude of the intermediate frequency signal is detected by a receiver 32 and the PIM measurement 34 is obtained.
Components of a measuring instrument can contribute delay to measurements obtained using the test system. For example, the filters 16, 26 of the measuring instrument 1 of FIG. 1 contribute a group delay that has associated with it an uncontrollable length. The uncontrollable length is substantial enough to corrupt a measurement of PIM, thereby preventing a usable determination of the distance to the PIM source from the measuring instrument (referred to hereinafter as distance-to-PIM). The PIM source cannot be readily located without a useable determination of distance-to-PIM. The measuring instrument can be calibrated to account for inherent delay caused by components within the measuring instrument. FIG. 2 illustrates a two step technique for calibrating a measuring instrument 1 such as shown in FIG. 1 using a calibration standard 50 generating a known PIM and having a known distance-to-PIM. The load 40 is attached to the test port of the measuring instrument 1 and measured. The load 40 is then removed and the calibration standard 50 is attached to the test port. The load 40 is then attached to the calibration standard 50 and the load is measured, along with the intervening PIM generated by the calibration standard 50. The measuring instrument 1 is calibrated based on the determination of distance-to-PIM by the measuring instrument 1, and the known distance-to-PIM of the calibration standard 50.